The use of semipermeable membranes for reverse osmosis or ultrafiltration process is well known in the art. For example, in a reverse osmosis process for the separation of liquids such as water, a pressure in excess of the osmotic pressure of saline water is applied to the feed solution which is placed in contact with the semipermable membrane. This membrane is permeable to water which will diffuse through the membrane while the sodium chloride molecules and other impurities which may be present in the water are retained by the membrane. The purified water which is recovered as the permeate may then be utilized for personal use such as cooking or drinking.
In addition to the use of semiperable membranes for the separation of liquids, it is now possible to use certain membranes for the separations of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feedstream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the gas mixture will pass through the membrane at a more rapid rate than will the less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
This ability to separate gases from a mixture stream has found many applications in commercial uses. For example, gas separation systems can be used for oxygen enrichment of air, for improved combustion efficiencies and conservation of energy resources. Likewise, nitrogen enrichment of air can be applicable where inert atmospheres are required. Other applictions for oxygen enriched gases may be improving selectivity and effciency of chemical and metallurgical processes. Similarly, inert atmospheres such as may be provided for by this invention may also be utilized in chemical and metallurgical processes. Some other applications of gas separation would include helium recovery from natural gas, hydrogen enrichment in industrial process applications, and scrubbing of acid gases. In addition, gas separation systems may be used for environmental benefits, e.g., methane can be separated from carbon dioxide in waste gases for sewage treatment processes and oxygen enriched air can be produced to enhance sewage digestion.
U.S. Pat. No. 3,842,515, discloses a process for preparing cellulose ester membranes which may be used for water desalination in a reverse osmosis process. The process for preparing these membranes involves immersing a water-wet cellulose ester membrane in a water-soluble alcohol until substantially all of the water in the membrane has been replaced by the alcohol. Following this, the alcohol-wet membrane is then further immersed in a non-polar alcoholsoluble organic liquid solution for a period of time which is sufficient to replace the alcohol with the organic liquid, following which the membrane is dried to produce the desired product.
Another U.S. Pat. No. 3,033,698 describes the use of titanium chelate compounds having the formula (RO).sub.x Ti(R').sub.4-X in which R is an alkyl radical and R' is an oxy compound capable of chelating with titanium for increasing the viscosity of a casting dope comprising cellulose acetate, acetone and water. The increase of the viscosity of the dope is to enable the production of fibers therefrom. The titanium chelates in addition to increasing the viscosity are also used to lightly cross-link the polymer. Inasmuch as only a light cross-linking of the polymer is required the titanium chelates are less reactive in nature. More reactive titanium chelates would result in a high cross-linkage of the polymer and thus turn the solution into a gel which can not be spun into fibers. In contradistinction to this, the present invention utilizing organic titanates will result in a high cross-linking of the cellulose acetate to enable the formation of membranes capable of being utilized for gas separation processes in which the porous membrane will be highly resistant to pressure and temperature. The spinning formulation utilized by the patent will include water and acetone. However the presence of water will adversely affect the degree of cross-linking when used with highly reactive titanates and therefore, the solution which is used in the present invention will not contain any water, the solvents being only organic in nature. Another distinction is that in the patent the polymer is essentially cross-linked in solution while in the present invention the polymer is cross-linked after being formed as a solid membrane.
As hereinbefore set forth, the separation of various gases from a mixture thereof may constitute an important advance in commercial applications. This is becoming increasingly important in view of the necessity to conserve energy. A particular application would relate to increasing the thermal efficiency of combustion processes when utilizing fossil fuels in commercial combustion applications. Also, by utilizing a gas separation membrane in coal gasification, it may be possible to provide an oxygen enrichment of air for the production of low and medium British thermal unit (BTU) product gases as well as an oxygen enrichment of air for the combustion of these gases. For example, by placing a gas membrane separation system in close proximity to both gas production and gas combustion facilities, it would allow a site-located oxygen enrichment plant to supply both processes without the additional expense of transporting the gas or duplicating enrichment facilities. It is also contemplated that the membranes may also be employed in a system utilized to scrub gases such as acid gases, H.sub.2 S, CO.sub.2, etc. from the system.
As will hereinafter be shown in greater detail we have now found that a cellulose acetate membrane which is gas selective in nature may be prepared by cross-linking the cellulose acetate with a particular type of cross-linking agent to produce a membrane which will posses increased temperature and pressure stability, resistane to moisture, and resistance to organic solvents.